Waste Water Safety Element Torque Limiter and Method of Construction

ABSTRACT

A resettable torque limiter in which major components are cast from stainless steel in order to minimize material requirements and which are thereafter annealed to provide corrosion resistance in service.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/279,134 filed Oct. 16, 2009, incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention concerns safely torque limiters used in wastewater treatment facilities.

Such facilities typically include rectangular settling tanks which havescrapers or flights connected by a drag chains which are driven byelectric motors and reducers. Safety overload torque limiters areincorporated in the drive to protect the drive equipment in case a jamcondition develops.

Simple shear pin designs are in wide use but are less reliable and morelabor intensive to maintain and create longer down times since they mustbe replaced when sheared. Failures due to the corrosive environment haveoften occurred with other designs.

A superior resettable torque limiter design has been developed for thisapplication, using a drive ball which is spring urged into a position bya plunger where it creates a driving connection between two rotarymembers in the torque limiter. The driveball is driven out of areceiving recess in one member to overrun when the torque exceeds apreset maximum. The drive ball will not resume its driving position whenthe torque level declines due to an arrangement which prevents thespring force from being active once an overrunning event has occurred.The torque limiter is reset by repositioning the drive ball into itsdriving position as by tapping the plunger with a hammer or mallet.

Such torque limiters have operated very reliably over many years inservice which is important as the waste treatment equipment is costlyand must be kept operational at all times.

The corrosive environment in which these torque limiters operate hasdictated that stainless steel be used in the manufacture of itscomponents and this causes that limiter to be relatively costly tomanufacture due to the considerable amount of expensive stainless steelwhich is required to manufacture these torque limiters.

Such torque limiters have conventionally been constructed from stainlesssteel bar stock which is rolled when produced, which rolling develops ametallurgy in the material which is highly corrosion resistant. Thus,use of bar stock for the torque limiter parts results in highlycorrosion resistant parts and provides a very good long term serviceperformance.

Using stainless steel castings has been regarded as unsatisfactory forapplications subjecting parts to corrosion as being much more subject tocorrosion due to the different metallurgy produced by the castingprocess. Such parts are likely to fail in a relatively short time whenused in water treatment facilities.

There is considerable stainless steel material lost in the machining ofthese large parts from bar stock, since it requires extensive machiningaway of the material, causing substantial waste of stainless steel intheir manufacture. The need for such extensive machining of those partsalso adds significantly to the cost of such torque limiters.

It is an object of the present invention to provide a corrosionresistant resettable safety torque limiter for waste water treatmentfacilities of the type described which is highly corrosion resistant tobe well suited to waste water treatment applications but which can bemanufactured at significantly lower cost.

SUMMARY OF THE INVENTION

The above recited object and other objects which will be understood upona reading of the following specification and claims are achieved byprecision casting the major torque limiter rotary components fromstainless steel, which allows the components to be cast close to theirfinal dimensions, eliminating the extensive machining operations whichwere previously required to produce such components and the waste ofstainless steel metal which is machined away.

In addition, recess features are preferably molded into these componentsat locations which will not affect their proper performance during theservice life of the torque limiter to achieve additional materialsavings.

The stainless steel alloy used is a low carbon alloy, i.e., 0.03% carbon316L (CF3M) stainless steel.

In order to achieve the necessary corrosion resistance, the castcomponents are annealed after casting by being heated to a temperaturein the range of 1850° F. to 2050° F. in an oxygen free atmosphereprovided by a vacuum furnace, with subsequent rapid quenching as in asand bed fluidized with nitrogen gas, preferably in a multiple chamberfurnace to achieve sufficiently rapid quenching.

This annealing process largely eliminates precipitate carbon which isbelieved to make the cast material subject to corrosion, and haveproduced corrosion resistant parts.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view through a safety element torque limiter oftype which the present invention is concerned.

FIG. 1A is a fragmentary enlarged view of a portion of the torquelimiter shown in FIG. 1.

FIG. 2 is an exploded pictorial view of the torque limiter shown in FIG.1.

FIG. 3 is an enlarged pictorial view of a module carrier hub componentincluded in the torque limiter shown in FIG. 1.

FIG. 4 is a pictorial view of a detent pocket plate included in thetorque limiter shown in FIG. 1.

FIG. 5 is a pictorial view of a retaining plate included in the torquelimiter shown in FIG. 1.

DETAILED DESCRIPTION

In the following detailed description, certain specific terminology willbe employed for the sake of clarity and a particular embodimentdescribed in accordance with the requirements of 35 USC 112, but it isto be understood that the same is not intended to be limiting and shouldnot be so construed inasmuch as the invention is capable of taking manyforms and variations within the scope of the appended claims.

Referring to the drawings, and in particular FIGS. 1 and 2, a resettabletorque limiter 10 is shown of the type described in U.S. 2010/0224713A1,incorporated herein by reference.

A carrier hub 12 has a relatively rotatable pocket plate 14 supportedthereon.

A retainer plate 16 is affixed to one end of the carrier hub 12 andtogether with a flange on the other end of the carrier hub 12 axiallycaptures the pocket plate 14 on the carrier hub 12.

A drive shaft (not shown) is received in a common bore 18 machined tothe size of a shaft to be received therein (not shown) in the retainingplate 16 and carrier hub 12, with a key groove 20 and set screw hole 22also provided in the well known manner.

An output member 24 is connected to be driven by the pocket plate 14.

A disconnectable drive unit 26 is mounted on a hub of the carrier hub 12and includes a primary drive ball 28 urged against a seat 30 carried bythe pocket plate 14 by a spring loaded plunger 32 so that the drive ball28 is positioned partially within aligned respective recesses in thecarrier hub 12 and the pocket plate 14 to establish a rotary driveconnection between the carrier hub 12 and pocket plate 14.

The torque transmitted from the carrier hub 12 through the drive ball 28to the pocket plate 14 tends to cam the drive ball 28 out of the recessin the pocket plate 14 and back into the recess in the carrier hub 12since the center of the ball 28 lies within the recess in the carrierhub 12 when on the seat 30. This movement is resisted by the Bellevillesprings 40 urging the plunger 32 and engaged drive ball 28 to the left.

There is a counterforce acting on the plunger 32 tending to move theplunger 32 back to the right generated by the torque transmitted by thedrive ball 28. When a predetermined maximum load is exceeded, a set ofsecondary balls 34 are moved by the force acting on the plunger 32 toride out on ramp ring 36 to an overrunning position on an outsidediameter 38 of the plunger 32, releasing the spring force normallyexerted on the ball 28 by the Belleville springs 40 (FIG. 1A).

This allows the drive ball 28 to be moved completely to the right out ofthe recess in the pocket plate 14 by the reaction to the torque actingbetween these components to disconnect the drive between the carrier hub12 and pocket plate 14. This continues until the drive unit 26 is resetas by driving the plunger 32 to the left to cause the balls 34 tore-enter the space between the ramp 36 and facing ring 37.

The major components of the torque limiter 10 are the carrier hub 12,the pocket plate 14 and the retainer plate 16, and each have portionsthereof that are only light stressed in service.

According to the invention, these parts are cast from an austeniticstainless steel, using a precision casting process such as investmentcasting to minimize the material to be machined away, saving on materialand machining time. The cast material is rendered corrosion resistant bya subsequent annealing process, known per se but not heretofore appliedto this application. A preferable material austenitic stainless steel islow carbon 316L stainless steel (CF3M), i.e. containing approximately0.03% carbon. The composition is as follows:

ASTM 351 316L (CF3M) C <0.03% Mn <1.50% Si  <1.5% P <0.04% S <0.04% Cr18.00%-21.00% Ni  9.00%-12.00% Mo 2.00%-3.00%

In the as cast state, carbon tends to be precipitated and formslocalized areas subject to initiation of corrosion.

By heating the casting to approximately 1900° F. (1850-2050° F.) thecarbon precipitate is largely eliminated by causing the carbon tore-enter solution.

The parts are then quenched to prevent the carbon from againprecipitating.

The quenching is preferably carried in a sand bed fluidized by nitrogengas, in a two or three chamber vacuum nitrogen back fill.

In FIG. 3, the carrier hub 12 is shown having two voids 42 cast thereinintermediate the two holes 44 provided to accept the overload releaseunits 26 (only one can be provided and one hole 44 covered by a coverplate 46). Full thickness material is maintained in the area surroundingthese holes 44.

In FIG. 4, the pocket plate 14 has four cast in voids 48 located betweenbolt holes 50 except where the pockets 56 (FIG. 1) are formed (on theopposite side) to receive the drive ball 28 and seat 30.

FIG. 5 shows voids 58 cast in between the bolt holes 60 except in thelocation aligned with the key way 20 and set screw hole 22.

Accordingly, a torque limiter suitable for waste water treatment driveequipment can be provided at a substantially lower cost than previousdesigns of such devices while insuring an extended service life.

1. A method of manufacturing a safety element torque limiter including:a carrier hub; a pocket plate rotatable on said carrier hub; a retainerplate fixed to said carrier hub and together with a flange on saidcarrier hub confining said pocket plate axially on said carrier hub; adrive ball held on a seat in a recess formed in said pocket plate by analigned spring loaded plunger mounted on a hub of said carrier hub so asto create a spring force acting on said plunger to thereby hold saiddrive ball on said seat, said drive ball positioned partially withinsaid recess in said pocket plate and a normally aligned recess in saidcarrier hub, said spring acting on said drive ball though a ramp ringand a set of interposed secondary drive balls which are moved up andpast said ramp as onto an outside diameter of said plunger when torqueexceeds a predetermined maximum level to discontinue exertion of saidspring force on said plunger and allow said drive ball to be forced outof said recess in said pocket plate and disconnect drive between saidpocket plate and said carrier hub; wherein said retainer plate, saidcarrier hub and said pocket plate are each precision cast from a lowcarbon stainless steel and thereafter annealed by heating to atemperature in the range of 1850° to 2050° F. and thereafter rapidlyquenched.
 2. The method according to claim 1 wherein said quenching iscarried out in sand fluidized by nitrogen gas.
 3. The method accordingto claim 1 wherein 316L stainless steel with 0.03% carbon is used incasting said retainer plate, carrier hub and pocket plate.
 4. The methodaccording to claim 3 wherein a series of voids are in cast into each ofsaid retainer plate, carrier hub and pocket plate intermediate holestherein.
 5. The torque limiter made by the method of claim 1.